Two engineering alloys, of the same overall yield strength (300 MPa) and elastic properties, are being considered as candidate materials for a particular structural application. Material 1 has a grain size of lOjim, while the grain size of Material 2 is 220 jim. Both alloys fail in an intergranular fracture mode by the nucleation, growth and coalescence of voids at grain boundaries. The structural component, for which the alloys are being considered, is subjected to a tensile stress of 85 MPa. It contains a through thickness single edge crack of length a = 25 mm. The thickness of the part containing the crack is 30 mm and its width is 100 mm. (The equations necessary to calculate the stress intensity factor for this problem can be found in the Appendix.)
(a) Comment on the extent and validity of /-dominance for the two materials under consideration.
(b) An examination of the ASTM standard (E813) for elastic-plastic fracture toughness testing of these two materials reveals the requirement that the depth of the initial crack (i.e. the notch length plus the length of the fatigue pre-crack) be at least one-half of the width of the specimen (e.g., compact tension, three-point bend specimen, or four-point bend specimen). Why?